A TETRATRICOPEPTIDE REPEAT MUTATION IN YEAST TRANSCRIPTION FACTOR IIIC131 (TFIIIC131) FACILITATES RECRUITMENT OF TFIIB-RELATED FACTOR TFIIIB70

Transcription factor IIIC (TFIIIC) plays an important role in assembli ng the initiation factor TFIIIB on genes transcribed by RNA polymerase III (Pol III), In Saccharomyces cerevisiae, assembly of the TFIIIB co mplex by promoter-bound TFIIIC is thought to be initiated by its tetra tricopeptide repeat (TPR)-containing subunit, TFIIIC131, which interac ts directly with the TFIIB-related factor, TFIIIB70/Brf1. In this work , we have identified 10 dominant mutations in TFIIIC131 that increase Pol III gene transcription, All of these mutations are found within a discrete 53-amino-acid region of the protein encompassing TPR2. Bioche mical studies of one of the mutations (PCF1-2) show that the increase in transcription is due to an increase in the recruitment of TFIIIB70 to TFIIIC-DNA. The PCF1-2 mutation does not affect the affinity of TFI IIC for DNA, and the differential in both transcription and TFIIIB com plex assembly is observed at saturating levels of TFIIIB70. This indic ates that mutant and wild-type TFIIIC-DNA complexes have the same affi nity for TFIIIB70 and suggests that the increased recruitment of this factor is achieved by a nonequilibrium binding mechanism, A novel mech anism of activation in which the TPR mutations facilitate a conformati onal change in TFIIIC that is required for TFIIIB70 binding is propose d. The implications of this model for the regulation of processes invo lving TPR proteins are discussed.